Speed control system for electric motors



Feb. 26, 1946. H. M. STOLLER 2,395,517

SPEED CONTROL SYSTEM FOR ELECTRIC MOTORS Filed Nov, 2l, 1942 2Sheets-Sheet 1 Feb. 26, 1946. H. M. STOLLER SPEED CONTROL SYSTEM FORELECTRIC MOTORS FiledNov. 21. 1942 2 Sheets-Sheet 2 /Nx/E/vrof? BVHMSTOLLER ATTORNEY Patented Feb. 26, 1946 SPEED CONTROL SYSTEM FORELECTRIC TORS Hugh M. Stoller, Mountain Lakes, N. J., assigner ,to Bell.Telephone Laboratories, Incorporated,

New York, N. Y., a corporation of New York Application November 21.1942, Serial No. 466,509

(ci. 1v1- 312) 23 Claims.

'I'his invention relates to speed control systems, and more particularlyto such systems including space discharge apparatus for controlling thespeed of electric motors. l

Heretofore, speed control systems embodying space discharge apparatuswere known for the purpose of regulating a motor so as to maintain thespeed thereof ksubstantially constant. Certain prior art systems of theforegoing type were disclosed'in my patents, Nos. 1,695,035 and1,711,661, granted December 11, 1928, and May 7, 1929,

respectively. Such speed regulating systems wereuseful in cases in whichthe factors of bulk and weight and effects of ambient temperaturelvariations were of no special concern. 'I'he present invention isparticularly applicable for use with electric motors whose physicallocations, such as in the ileld of mobile apparatus, require that bulkand weight be kept down and that the eiects of ambient temperaturevariations be reduced.

The present invention contemplates a speed control system of theelectron discharge tube type in which precision and stability areimproved over a predetermined range of variation of ambient temperatureand in which weight and bulk are both reduced.

In a specific embodiment the present invention comprises an electricmotor whose speed is to be controlled and which includes a regulatingfield winding, and a generator operated by the motor to produce a pilotalternating current whose frequency is proportional to the speedthereof. A frequency discriminating bridge comprising three physicalarms, of which one is tuned to a certain frequency corresponding to thenormal speed of the motor, has a pair of input terminals applied to thepilot 'wave generator and a pair of output terminals connected to theinput of a phase detector tube whose output is also applied to the pilotwave generator and which embodies a resistor. 'I'his resistor isconnected to the control grid of a power tube whose output includes themotor regulating field winding.

In the operation of the speciilc embodiment of the present invention,the pilot generator supplies pilot waves to the input terminals of thebridge at whose output terminals occurs an output wave having amagnitude and phase depending on the variation ofthe frequency of thepilot wave with reference to the certain frequency to which the onebridge arm is tuned. When the pilot wave has, in response to normalmotor speed, the certain frequency, a balanced condition is establishedin the bridge and no output voltage occurs at its output terminals. Thismeans that no adjustment of the amount of current flowing in theregulating field winding of the motor takes place. l

When the frequency of the pilot wave is Labove normal, the bridgebecomes unbalanced to produce an output voltage of one sign and when thepilot wave frequency is below normal the bridge becomes unbalanced inthe opposite direction to produce an output voltage of opposite sign.The pilot wave is applied to the plate circuit of the detector tubewhile the bridge unbalance voltages are applied to the grid of thistube. When the two waves are in phase the eiect is to make the gridrvoltage of the power more negative, `thus weakening the regulationfield; and when-the twov Waves are opposite in sign the opposite effectis produced on the power tube, thus strengthening the regulation field.In this way speed regulation is effected.

A feature of the invention is a regeneration or feedback circuitextending between the phase detector and power tubes such that thecurrent flowing in the motor regulating iield winding also ows through aresistor which is included in the biasing circuit for the control gridof the phase detector tube. Hence, changes in the amount of currentflowing in the motor regulating ileld winding are instantaneouslyvreiected as corresponding changes in the effective biasing potentialapplied to the control grid of the phase detector tube so as to controlfurther the amount of space current flowing in the latter tube. Thisprovides for the motor a substantially flat speed versus primaryvariable characteristic.

Another feature is that as the resonant bridge arm is effectively a pureresistance at the certain frequency to which it is tuned, this resist--ance tends to undergo change of magnitudewhen subjected to thepredetermined range of ambient temperature variation. The principalcause of such variation is the changes of the direct current resistanceof the copper wire winding of the inductance embodied in the resonantbridge arm. This causes changes in the balanced or unbalanced conditionof the bridge. To compensate such tendency, the bridge network includesin another arm a resistance network embodying a thermistor having apreselected temperature coefiicient of resistance for the predeterminedrange of variation of ambient temperature. As a consequence, apredetermined ratio of resistance between the resonant and resistanceyarms, is maintained substantially constant over the predetermined rangeof ambient temperature variation.

Another feature relates to the production of a transient output by thebridge network to exert a compensating effect on transient conditions inthe system which transient conditions would tend to interfere with theregulation of speed.`

motor. As a consequence, no auxiliary starting device is required forthe motor. An additional feature involves the control of the speed ofthe motor in synchronism with the fixed characteristic of an elementunaffected by the operation of the motor whereby a substantialimprovement in precision is obtained. Another feature concerns the useof a physical four-arm bridge in place of the physical three-arm bridge.A further feature relates to filter networks for attenuating harmonicsof the pilot wave while at the same time compensating for the inherentphase characteristic of the frequency discriminating bridges.

The invention will be readily understood from the following descriptiontaken together with the accompanying drawings, in which:

Fig. 1 is a schematic circuit diagram illustrating a specic embodimentof the invention;

Fig. 2 is a family of curves illustrating certain action in Fig. 1;

Fig. 3 is a schematic circuit diagram of a crystal controlled oscillatorfor effecting synchronous regulation in Fig. 1; `and Fig. 4 is aschematic circuit diagram of a Wheatstone bridge which may be utilizedin Fig. 1.

The same reference numerals are employed to identify the same elementsappearing in Vthe several figures of the drawings.

Referring to Fig. 1, a dynamotor l embodies a series field Winding 9, ashunt field winding Il, a regulating field winding i2, and a pilotwinding I3. A toothed rotor I4 also embodied in the dynamotor I0constitutes, together with the winding i3, a generator of a pilotalternating wave as disclosed in my copending application, Serial No.450,037, iiled July '7, 1942. The pilot generating Winding i3 isconnected to an electrical winding 25 which is inductively coupled toboth electrical windings 2i and 22. A capacitor 23 is disposed acrossthe winding 2l for a purpose that will be hereinafter pointed out.

A frequency discriminating bridge network 3l disclosed in my copendingapplication, Serial No. 466,507, filed November 2l, 1942 comprises ai'lrst arm 32 embodying a molybdenum-permalloy core 33 onto which isapplied an electrical winding 34 to constitute an inductance coil which,together with a capacitor 35 connected in parallel therewith,constitutes a resonant circuit tuned to a certain frequency which willbe hereinafter identified; a second arm 35 connected to the first arm 32and provided with a resistance network comprising a fixed resistor 31arranged in series with a fixed resistor 38 and a thermistor 39 disposedin parallel; and a third arm 45 comprising an electrical winding 4|applied to the core 33 and coupled inductively to the electrical winding34 wound thereon. The fourth electrical arm of the bridge comprises themutual inductance between the coupled windings 34v and 4|.

asomar? Corresponding terminals of the bridge arms 99 and 40 constitutebridge terminal 28 which is extended through resistor 45 to the inputwinding terminal 25, and one terminal of the resonant bridge arm 32constitutes bridge terminal 29 which is extended through resistor 41 tothe input winding terminal l21. Across the bridge ends of theseresistors is applied a. capacitor 48. The output terminals of the bridgenetwork 3l consist of one terminal 42 extended to both a. ground point43 and a point 44, which is common to a junction of both the respectivefirst and second arms 32 and 36, and another terminal 45 applied to thefree end of the bridge arm 40. The resistors 46 and 41, together withcapacitor 48, and a capacitor 49 disposed across the bridge outputterminals 42 and 45 and coupled with the internal resistance of thebridge network 3| constitute nlter and phase changing networks.

The bridge network 3| has its output terminal 45 extended over a lead 55embodying a resistor 55 to the .control grid of a phase detector tube 51whose cathode is connected over lead 58 and resistor 59 to a point 50 atground potential. Thus, the input circuit of the phase detector tube 51comprises ground point 43. bridge output terminal 42, capacitor 49,bridge output terminal 45, resistor 55, lead 56, control grid-cathodecircuit of the phase detector tube 51, lead 58, resistor 59 and groundpoint 80. The output circuit of the phase `detector tube 51 includes itsanodecathode circuit, lead ,58, point 63, resistors 58 and 51 in series,electrical winding 22 coupled inductively to the pilot wave winding 20,resistor 68 and back to the anode of the phase detector tube 51.Resistor 69 has one terminal connected to the junction point ofresistors 66 and 61 and its opposite terminal extended over leads 10 and10a to brush 1I engaging the 25-yolt commutator of the dynamotor l0., Acapacitor 15 is disposed across the serially con nected resistors 66 and51.

The 25-volt commutator has its brush 12 extended through series winding9 and over leads 83a and 13 to ground point 14. The primary line voltageof 25 volts is applied between lead 83h connected to the brush 1| andlead 83a extended through series winding 9 to the brush 12. Theresistors 59, 66 and 69 constitute a potentiometer which suppliesnegative biasing voltage to the control grid of the detector tube 51.The effective negative biasing voltage lmpressed on the control grid ofthe detector tube 51 is the voltage produced across the cathode resistor59 and comprises two portions: (a) the portion due to the current flowin the circuit comprising dynamotor brush 1|, leads 10a and 10,resistors 69, 65 and 59, ground points B0 and 14, leads 13 and 83a,series winding 9, and dynamotor brush 12 to the 25-volt commutator ofthedynamotor I0; and (b) the amount of space current owing in aregenerative feedback circuit to be explained later herein. The shuntfield winding I I is extended over leads 13a and 10a to the brush 1|engaging the 25- volt commutator of the dynamotor I0.

A point 15 applied to corresponding terminals of the resistor 61 andcapacitor 'l5 in the output circuit of the phase detector tube 51 isextended over leads 11 and 18 and resistor 18a to the control grid ofpower tube 19 whose cathode is connected over lead a to the terminal 53common to the input and output circuits of the phase detector tube 51.Thus, the voltage produced across resistors 59 and 91 is impressed onthe of the pilot wave supplied to 19. Hence, the voltage applied to thecontrol ohms compared with resistor 61o!V grid of tube 19 and influencedby the output tube 51 will be hereinafter referred to as the voltageproduced across the resistor 61. The anode of the power tube'19 overlead 80; regulating field winding |2, dynamotor brush 8|, which is alsoapplied to the screen grid of the power tube 19, Z50-volt commutator ofthe dynamotor I0, dynamotor brush 82, leads 83, 33a and 13 to the groundpoint 14. The potentiometer comprising resistors 59, 66 and 69, servesto apply a fixed positive biasing potential determined by the resistor66 to the control grid of the power tube 19.

The regenerative feedback circuit mentioned above extends betweenfthepower tube 19 and phase detector tube 51 and comprises the anodecathodeof the power tube 19, lead 80a, point 63, resistor 59, ground points 60and 14, leads 13, 83a and 83, brushes 82 and 8| engaging the 25o-voltcommutator of the dynamotor I0, regulating field winding |2, and lead 90extended to the anode of the power tube 19. This circuit serves to varythe direct biasing voltage produced across the resistor 59 in responseto changes occurring in the circuit of amount of current flowing in theregulating field winding |2 in a manner explained.

A capacitor junction point 86 of the leads 11 and 18 which connect theresistor 61 to the control grid of the power tube, and its oppositeterminal joined to one end of a resistor 81 whose opposite terminal isextended to the ground point 14. The capacitor 85 and resistor 81arranged in series constitute a damping network which functions toprevent hunting in the system of Fig. 1 thereby -to establish certainamplification in the power tube 19 in the manner explained in thecopending application, Serial No. 466,508, of E. R. Morton and H. M.Stoller filed November 21, 1942.

The operation of Fig. 1 will now be explained. When a pilot wave of thecertain frequency to which the first bridge arm 32 is tuned is presentin the windings 20 and 2| and is thereby applied across the bridge inputterminals 28 and 29, a condition of balance is established in the bridgenetwork 3| such that no alternating output wave is caused to occuracross the bridge output termials 42 and 45. When the pilot wave has adifferent frequency anced, and an alternating output wave is producedacross the bridge output terminals 42 and 45. This output wave isapproximately in phase with the pilot wave present in the windings 20and 2| if the frequency of the pilot wave is lower than the frequency atwhich the bridge is balanced, and is approximately 180 degrees out ofphase with reference to the pilot wave present in kthe windings 20 and2| if the frequency of the pilot wave is higher than the frequency atwhich the bridge is balanced. Thus, as the frequency the windingterminals 26 and 21 varies from above to below the certain frequency, orvice versa, the relative phases of the output waves produced across thebridge output terminals 42 and v45 vary through 180 degrees; and

that will be hereinafter is extended- 85 has one terminal connected toawill be assumed that the waves vary in proportion to the departure ofthe frequency of the pilot wave from the certain frequency to which theresonant arm 32 is tuned.

The circuit of Fig. l is initially adjusted to operate so that at normalspeed the pilot wave supplief *c the coupled windings 20, 2| and 22 hasa frequency which is equivalent to the resonant or certain frequencytuned. This means that no alternating output wave is established acrossthe bridge output terminals 42 and 45, and consequently, no alternatingvoltage is impressed on the control grid of the.

phase detector tube 51. Further, this means that the fixed amounts ofdirect biasing voltage applied to the control grids of the phasedetector tube 51 and the power tube 19 in the manner hereinbeforedescribed cause `a normal amount of current to flow in the regulatingfield winding |2 whereby the speed. is regulated to the normal value.For the purpose of this illustration, it

frequency of the generated pilot wave is 120 cycles per second at anormal speed of 1200 revolutions per minute; the frequency of thegenerated pilotywave is proportional to speed; and the resonant bridgearm 32 is tuned to the frequency of 720 cycles per second.

Let it be assumed now decrease below normal due, for example, to anincrease in load or a decrease of line voltage. As a consequence, thefrequency of the pilotwave present in the windings 20, 2| and 22 tendsto bbth windings 2| and 22 the bridge network 3| is unbalthe magnitudesof such output decrease. The application of a portion of the pilot waveof the decreased frequency to the input of the frequency discriminatingnetwork 3| via the coupled windings 20 and 2| causes across its outputterminals 42 and 45 the occurrence of an output wave whose magnitude isproportional to the extent 'that the '720.cycles per second and which isapproximately in phase with the portion of the pilot wave present in thewinding 2|. This output wave is impressed on the control grid of thephase detector tube 51 over the above-identified input circuit therefor.

At the same time another portion of the pilot wave is supplied to theoutput circuit of the phase detector tube 51 via the coupled windings 20and 22. As the portions of the pilot wave present in are in phase, thenthe phase ofthe wave impressed on the control grid of the phase detectortube 51 is in phase with the portion of the pilot wave of. the decreasedfrequency impressed on the output circuit of the latter` tube. Thus, theoutput wave supplied to the control grid of the phase detector tube 51is positive when the wave supplied to-the anode thereof is positive, andtherefore the amount of space current flowing in the latter tube iscaused to increase. This causes a corresponding increase in themagnitude of the direct voltage produced across the resistor 61 in theoutput of the phase detector tube 51. Hence, the direct biasingpotential impressed on the control grid of the power tube 19 is renderedmore negative. This occasions a decrease in the amount of space currentflowing in the power tube 19, and thereby a corresponding decrease inthe amount of current flowing in the regulation winding |2 of the'dynamotor I0. This tends to restore the speed to normal.

Let it be assumed now that the speed tends to increase above normal due,for example, to a-decrease in load or an increase of line voltage. As aconsequence, the frequency of the pilot wave present in the windings 20,2| and 22 tends to increase. The application of a portion of the pilotto which the first arm 32 is that the speed tends to,

frequency of the pilot is below wave of the increased frequency to theinput of frequency discriminating network 3| via the coupled windings 20and 2| will cause at its output terminals 42 and 45 the occurrence of anoutput wave whose magnitude is proportional to the extent that thefrequency of the generated pilot wave is above 720 cycles per second,and,l which is approximately 18|) degrees out of phase with reference tothe portion of the pilot wave present in the winding 2 I. This outputwave is impressed on the control grid of the phase detector tube 51 overthe previously traced input circuit therefor.

At the same time another portion of the pilot wave is supplied to theoutput circuit of detector tube 51 via the coupled windings 20 and 22.As the portions of the pilot wave present in both windings 2| and 22a're in phase, then the phase of the output wave impressed on thecontrol grid of the phase detector tube 51 is approximately 180 degreesout of phase, or leads in phase by approximately 180 degrees, withreference to the portion of the pilot wave impressed on the outputcircuit of the phase detector tube 51. Thus, the

wave supplied to control grid of the phase detector tube 51 is negativewhen the wave supplied to the anode thereof is positive, and thereforethe amount of space current iowing in the phase detector tube 51 iscaused to decrease. This causes a corresponding decrease in themagnitude of the direct voltage produced across the resistor 61 in theoutput of the phase detector tube 51. Hence, the direct biasingpotential impressed on the control grid of the power tube 18 is renderedless negative. 'I'his results in an increase in the amount of spacecurrent flowing in the power tube 19, and therefore a correspondingincrease in the amount of current flowing in the regulating 'winding I2.This increase of field strength tends to restore the speed to normal.

As pointed out in my copending application relating to the frequencydiscriminating network 3 I, supra, the resistors 46 and 41, togetherwith the capacitors 4B and 49, constitute filter and phase changingnetworks effectively operating as follows: (l) to attenuate harmonics ofthe pilot wave present in the windings 20, 2| and 22 such that onefilter stage comprises resistors 46 and 41 coupled with capacitor 48,and another filter-stage comprises capacitor 49 coupled with theinternal resistance of the bridge 3|; compensating phase shift ofapproximately 90 degrees between the input and output of the bridge 3|so as to offset the approximate Sil-degree phase shift which is inherentin the bridge 3| as indicated by Equations l and 2 of myabove-identified copending application relating to the frequencydiscriminating network 3 I. Thus, the output voltage eective across theoutput terminals 42 and 45 of the bridge 3| is rendered independent ofboth harmonics and the inherent phase characteristic Vof the bridge 3|.

Resistor 68 disposed in the output circuit of the phase detector limitsthe peak value of phase detector tube 51 to feedback current via thecoupled windings 2| and 22, terminals 28 and 29 of the bridge 3|, bridge3| to the input circuit of the phase detector tube 51 The bridge 3|embodying windings 34 and 4| on the common core 33 providesapproximately twice the output over the well-known four-arm Wheatstonebridge which will be hereinafter discussed in connection with Fig. 4.

e regenerative feedback circuit extending and (2) to provide a f vtheamount between the phase detector tube 51 and the power tube 10 andincluding the cathode biasing resistor 59 for the phase detector tube 51as hereinbefore identified provides feedback action between these tubesas follows: When the increased amount of current is caused to flow inthe regulating field winding I 2 in response to an increase in the speedas above amount of current also flows in the previously tracedregenerative feedback circuit so as to return to the ground point 5|)through the cathode biasing resistor 59. This increases correspondinglythe magnitude of the potential produced across the cathode biasingresistor 59, and thereby increases the magnitude of the effectivenegative biasing potential applied to the control grid of the phasedetector tube 51. This results 1n a further decrease in the amount ofspace current flowing in the output of the phase detector tube 51, andthereby a corresponding decrease in magnitude of the voltage producedacross the resistor 61 whereby the control grid of the power tube 19 isrendered still less negative. This causes a proportionate increase inthe amount of current flowing in regulating winding l2.

When a decreased speed causes a decrease of field current in regulatingwinding I2, this feedback circuit operates in the same manner to cause aproportionate further decrease in current in the regulating winding I2.

The result of the regenerative action is such that regardless of arelatively wide variation in of current supplied to the regulating fieldwinding I2 in response to changes in speed due to changes in linevoltage and/or load as above described, an equilibrium speed isultimately established at a value which is approximately equivalent tothe normal speed. Thus. the regenerative action provides a speedcharacteristic which fiat with reference changes in such primaryvariables as load, line voltage, or ambient temperature. In the absenceof the regenerative action, such characteristic would embody a finiteslope.

Referring to Fig. 2, the above-described operation of the speedregulator circuit of Fig. 1 will be readily understood. The amount ofcurrent flowing in the regulating field winding I2 is re1- rent flowingin the regulating field winding I2 falls gradually to zero. 'This causesthe dynamotor I 0 to accelerate to the normal speed of 7200 revolutionsper minute at which the amount of current flowing in the regulatingfield winding I2 rises substantially vertically to maximum, and remainsapproximately thereat regardless of further increases in the speedbeyond 7200 revolutions per minute. This current serves to bring thespeed back substantially to normal (7200 revolutions per minute) if forany reason the speed should increase thereover.

The amount of current flowing in the regulating field winding I2 dependsupon the magnitude of the space current flowing in the output circuit ofthe phase detector tube 51, which latter current determines themagnitude of the voltage explained, this increased detector tube 51 isrelatively below 4000 revolutions per minute; and attains a maximum lnthe range from 6800 to 7200-revolutons pel'v minute; and fallssubstantially vertically to zero -in the range from 7200 to '7500revolutions per minute. For higher speeds, this current remainsnegligible. V

The curve of detector plate current (tube 51) versus speed is due toboth the bridge output (network 3|) versus speed characteristic and thepilot generator output versus speed characteristic. Due to the factlthat the internal inductance of the dynamotor is coupled through'thetransformer windings 20 and 2| with the capacitor 23, these elementsconstitute a low Q parallel tuned circuit which d evelops maximum pilotgenerator voltage at a dynamotor speed of the order of three-fourthsnormal (approximately 5400 revolutions per minute). Thereafter, thisvoltage gradually falls off. It will be noted in Fig. 2 that the bridgeoutput voltage may be subdivided into an in-phase and a 90-de'gree or jcomponent. These are further discussed in my copending applicationrelating to the frequency discriminating network 3|, supra. At-speedsextending approximately from 5500 to 8500 revolutions per minute, thebridge output voltage (bridge 3|) furnishes the principal control of thespeed. In this connection it will be further noted that the detectorplate current (tube 51) follows substantially the 'j component of theoutput voltage of the bridge 3|, rather than the root mean square.

value of the latter voltage. The fact that the 7' component is positiveat speeds below 7200 revolutions per minute, then passes through zerosubstantially at the normal speed, and thereafter is negative at speedsabove '1200 revolutions per minute provides the desired characteristicof the output current of the phase detector tube 51. 'I'he droopingspeed voltage curve of the pilot generator cooperates with the bridge 3|in producing the desired characteristic of the output embodies no timedelay as previously pointed out. As the absence of time delay normallytends to cause hunting, or the oscillation of the circuit of Fig. 1about the normal speed, such hunting action is obviated due to astabilizing influence of the bridge network 3|, which stabilizing actionwill now be explained with reference to the effect of transients on thespeed.

Under the influence of a sudden change in line voltage or mechanicalload, the bridge network 3| yields a transient output voltage asexplained in my copending application relating to the frequencydiscriminating network 3|, supra. This transient output voltage issuperposed on the steady state output voltage hereinbefore described.Let it be supposed that a sudden increase occurs in the load. For thepurpose of distinguishing between transient and steady state or normalphenomena, let it be further Supposed that the increased load causes nopermanent4 change in the speed since the regenerative feedback actiondescribed hereinbefore functions to bring the speed back substantiallyto normal and that the only efi'ect of the increased load ls to cause adropping back in phase of the armature in a manner similar to thedropping back in phase of the armature of a standard type of chronousalternating current motor. As a result current of the phase detectortube 51. In the speed region above 8500 where the bridge outage preventsthe output current of detector tube 51 from rising. As a consequence thedesired regulating field rcurrent characteristic, Fig. 2, is obtained sothat the optimum field current is provided at all-speeds'over the entirerange from starting to any overspeed condition.

The Q of the winding 34 on the core 33 is so proportioned that thebridge 3l is most effective over a speed range equivalentl to x20 percent of the 'frequency of the generated pilot wave as mentioned in mycopending application relating to the frequency discriminating network3|, supra. the effects of the generated pilot; wave voltage and the jcomponent of the output voltage of the bridge 3|, the circuit of Fig. ldoes not re- Due to the speedv control exercised by quire an auxiliarystarting arrangement for the dynamotor: It has been found that despitethe stalling of the dynamotor, or purposely running the latter abovenormal speed, the regulator of Fig. 1 serves to cause the dynamotor toquickly return to the normal speed.

In a particular case, with a normal speed of A'1200 revolutions perminute, the actual speed was held within i2 revolutions per minute overa range of variation of line voltage from 20 to 30 volts and/or no loadto full load. In this connection, it is noted that the action of theregenerative feedback circuit is substantially instantaneous for thereason that the circuit of Fig. 1

of this supposition, the bridge 3| is balanced both at the conditions ofthe original and increased loads. In the interim, however, between suchbalancing of the bridge network 3|, a transient output is supplied tothebridge output terminals 42 and 45. This transient output is due to thefact that the resonant arm 32 of the bridge network 3| containselectrically stored energy in the form of 1/2 Llf-l-l/Z CE2; Where L isthe inductance of the winding 34, I is the current in amperes in thewinding 34, C is the capacity 0f the capacitor 35, and E is the voltageacross the resonant arm 32.

This transient output will be maintained for a time interval equivalentapproximately to the time of.4 or 5 cycles (with a coil having a "Q" ofthe order of 20). The transient output wave will be approximatelyl inphase with reference to the pilot wave present in the winding 22. Hence,the waves applied simultaneously to the input and output of the phasedetector tube 51 will also be in phase. Consequently, the transientoutput wave serves to substantially increase the amount of space.current flowing in the output circuits of the phase detector tube 51 andpower tube 19, at the start of the sudden change of the line voltage orload. This eifect is of value in, overcoming the unavoidable -lag due tothe time constant where r and L are the respective resistance and Iinductance of winding 34.

The control circuit of Fig. 1 does not wait until the speed has `fallento begin corrective action, but delivers a substantial counteractingtorque through tbe change in amount of current flowing in the regulatingfield winding I2 at the instant the change occurs in the line voltage orload. The transient output may have an initial amplitude which isseveral times larger than the maximum amplitude of the steady stateoutput wave of the bridge network 3l, but the transient outputdissipates itself after the time interval equivalent approximately tothe time interval of 4 or 5 cycles thereof, leaving only the steadystate output, if any, at the bridge output terminals 42 and 45 forcontrolling the speed in response to slow changes in line voltage andload as hereinbefore pointed out. As a consequence, the regulatorcircuit of Fig. 1 will compensate for both sudden and slow changes inload and/or line voltage and provide iiat regulation and a high degreeof stability.

Let it be supposed that a sudden decrease occurs in the load. As in thecase of the increase in load, the bridge 3l is balanced at theconditions of the initial and decreased loads; and the only effect ofthe decreased load is an advance in phase of the armature. In theinterim, however, a transient output is supplied to the bridge outputterminals 42 and 45. This is again due to the stored energy in theresonant arm 32 of the bridge 3i as above explained. This transient, asin the case of the previous transient, will exist for a few cycles. Thetransient output will be substantially 180 degrees out of phase with thepilot wave present in the winding Y22. Hence, the waves appliedsimultaneously to the input and output of the phase detector tube 51will be substantially 180 degrees out of phase. Consequently, thetransient output wave serves to decrease the amount of space currentflowing in the phase detector tube 51 and increase the current in powertube 19 substantially in an instantaneous manner, and thereby the amountof current flowing in the regulating field winding i2 as abovedescribed.

As the bridge arm 32 is essentially a pure resistance at its resonantfrequency of 720 cycles per second, its effective resonance resistancemay tend to vary with changes of ambient temperature. This would tend tocause the bridge 3| to show an output Voltage although the pilot wavebeing supplied thereto had a frequency of 720 cycles per second. Suchresistance variation of the bridge arm 32 is compensated for by thebridge arm 36 which includes the thermistor 39 provided with the propernegative temperature coefficient of resistance and whose effectiveresstance varies in a sense to counteract the changes of the resonanceresistance of the bridge arm 32 over a range of +40 C. to +140 C.variation of ambient temperature, for example, as pointed out in mycopending application relating to the frequency discriminating bridge3i, supra. As a consequence of this compensation, a predetermined ratioof the order of 1 to 1 for the purpose of the present illustration,between the effective resistance of the bridge arms 32 and 36 ismaintained substantially constant over the range of ambient temperaturevariation extending from +40 C. to +140 C.; and any tendency for avoltage to appear at the output terminals 42 and 45 of the bridge 3i inresponse only to the effect of such range of ambient temperaturevariation on the bridge arm 32 is substantially obviated.

The arrangement of Fig. 1 embodies an improvement of the order of to 1over prior speed regulators of the types disclosed in my two patents,supra, in the respects of both precision and stability. In other words,the system of Fig. 1 provides a regulation of speed of the order ofone-quarter of 1 per cent with the tubes i1 and 19 being of thecommercial type. This regulation can be improved to one-tenth of 1 percent over the same range of voltage, load and temperature variation butusing specially selected tubes 51 and 19. At the same time the weightand space occupied by the prior regulators have been so reduced that,for example, the equipment of the present invention weighs approximately1.5 pounds as compared to pounds for the prior regulators according tomy two patents, supra; the space occupied by the present invention isone-twentieth of a cubic foot as compared to two cubic feet of the priorregulators in accordance with my two patents, supra; and the cost issubstantially reduced.

A crystal controlled oscillator of the type shown in Fig. 3 may beemployed to introduce into the input of the phase detector tube 51 avoltage ixed as to both frequency and magnitude. In this connection itis understood that the circuit portion shown above the line X-X in Fig.3 is to be substituted for the circuit portion shown above the line X--Xof Fig. 1. Fig. 3 is an oscillator circuit of the crystal controlledtype, and comprises a resistance bridge in which a crystal 9| isdisposed in one arm, a thermistor 92 with the proper temperaturecoemcient of resistance is located in a second arm. and individualresistors 93 and 93a of fixed resistance characteristic are interposedin each of the remaining two arms. Space discharge tubes 94 and 95 areamplifiers. One diagonal of the bridge 90 is supplied from the output ofthe tube 95 through a capacitor 96 which blocks the direct voltageimpressed on the anodes of both the tubes 94 and 95 from source 91 ofdirect voltage. The input to the grid circuit of the tube 94 is obtainedfrom the other diagonal of the bridge As the output current of the tube95 is gradually built up to a certain level, the thermistor 92 is alsogradually heating up at the same time whereby the bridge 90 is graduallybrought into a balanced condition. As this is being done, the input tothe tube 94 is being gradually reduced. Ultimately, the level of theoutput current in the tube 95 is stabilized at a predetermined value.This loutput is applied to a buffer amplifier 91a whose output circuitincludes resistor 99 and ground points 99 and |00. The voltage developedacross the resistor 98 is introduced in the input circuit of the phasedetector tube 51 in series with the bridge output terminals 42 and 45.

'Ihe frequency of the oscillator in Fig. 3 is fixed at 720 cycles persecond, which frequency corresponds to the normal frequency of the pilotgenerator (winding I3 and rotor I4 of the dynamotor I9) as abovementioned; and the magnitude of the voltage produced across the resistor99 is fixed at a predetermined value. According to Figs. i and 3, theeifective voltages applied to the control grid oi' the phase detectortube 51 may be added in series as follows: (a) direct current `biasvoltage across resistor 59 in Fig. 1; (b) output voltage of the bridge3| as represented by the charge on capacitor 49 applied across itsoutput terminals 42 and 45; and voltage produced across the resistor 98in Fig. 3.

In the operation of Figs. i and 3, the voltage -across the resistor 99,Fig. 3, exerts on the average a negligible efi'ect at all speeds, exceptthe assumed synchronous speed of 7200 revolutions per minute. Thus, thedynamotor is started and brought up to the synchronous speed in themanner explained above in connection with Fig. 1. When the synchronousspeed is attained, the regulator of Fig. 1 is lockedi in with thestandard wave supplied bv the crystal oscillator, Fig. 3. At thesynchronous sp'eed, the pilot wave assumes a phase angle with referenceto the standard wave, which angle is about 90 degrees. An increase inthe load or a decrease in the line voltage causes a momentary decreasein the speed whereby the angle 0 is decreased. This tends to bring thepilot wave more nearly into phase with the standard wave whereby theoutput current of the phase detector tube 51 is increased so as todecrease the amount-of current flowing in the regulating field windingI2 to prevent a further decrease in the speed as abovey described. Thecompensating resistor 59 in combination with the damping networkcomprising capacitor 86 and resistor 81 restores the amplification ofthe power tube 19 to the new steady state condition, so that the angle 0resumes substantially its initial value approximately 90 degrees. Theaction of this damping network is explained in detail in the copendinglapplication of E. R. Morton et al., supra.

During the time interval when the angle 0 is decreasing, theinstantaneous frequency of the pilot wave is less than the frequency ofthe standard wave by an amount which amount is proportional to d dt orthe slope of the angle 0.` Referring to Equation 2 of my copendingapplication relating to the frequency discriminating network 3|, supra,

it will be noted that, when the frequency of thev pilot wave is belowthe certain frequency fo (720 cycles per second), the bridge 3| delivers-to its output terminals 42 and 45 an output voltage which voltage isAapproximately in phase with the pilot voltage present in the winding 22and applied to the output current of the phase detector tube 51. This,as previously pointed out, causes a decrease in the amount of currentflowing in the regulating ileld winding I2 whereby the tendency for thespeed to change is compensated. Thus, the action of the bridge 3| is insuch sense as to oppose a decrease in the angle 0, and thereby tends tostabilize further the regulator of Figs. 1 and 3.

A momentary decrease in the load or an increase in the line voltage,tends to cause an increase in the speed whereby the angle 0 isincreased. This tends to move the pilot wave further out of phase withreference to the standard wave whereby the output current of the phasedetector tube 51 is decreased so as to increase the amount of currentflowing in the regulating ileld winding I2 to prevent a. furtherincrease in the speed in the manner mentioned hereinbefore. 'I'hecompensating resistor 59 and the network `comprising capacitor 35 andresistor 81 restore the certain amplification of the power tube 19 tothe new steady condition, and the angle o returns substantially to itsinitial value of approximately 90 degrees.

During the interval when the angle o is 1n wave, Fig. 3. By using acrystal of the flexure tially equal resistors III.

type in the audio frequency range of 720 cycles per second, thefrequency of the standard oscillator can be made to match the slotfrequency of the dynamotor I Il. Such a crystal is physically small,mechanically permanent, and most precise as to frequency. By envelopingsuch crystal in a thermostatically controlled enclosure, precision ofthe order of three parts in a million is obtainable with the combinedcircuits of Figs. 1 and 3; and omitting the thermostatically controlledenclosure in the combined circuits of Figs. 1 and 3, precision of theorder of one part in ten thousand, or 0.01 per cent, is obtainable withan ambient temperature variation of 50 C. over the range abovespeciiled.

Fig. 4 shows a Wheatstone bridge ||0 embodying four physical arms whichmay be substituted in Fig. 1 for the bridge 3| embodying three physicalarms. In this connection it is understood that the circuit portion shownabove the line Y-Y in Fig. 4 is lto be substituted for the circuitportion shown above the line Y--Y inFig. 3; and further that thesynchronous operation effected by use of the crystal controlledoscillator of Fig. 3 may be accomplished with Figs. l and 4 by theproper substitution of Fig. 3 in Fig. 4 with respect to the line X-X.The bridge I0 embodies the resonant arm 32 and the resistive arm 3,6,and, for purpose of simplicity, two other arms which are substan- It isunderstood that the lower right arm II in Fig. 4 could also be anelectrical winding applied to the core 33 and cou-v pled to the winding34 as shown in the bridge 3|, Fig. 1 Thel operation of the bridge III),Fig. 4, is

substantially the same as that of bridge 3|, Fig.

1, except that it has reduced sensitivity with reference to the latterbridge.

What is claimed is:

l. A speed regulator for an electric motor, coniprising means togenerate an alternating wave whose frequency is proportional to thespeedof said motor, a bridge having a resonant arm tuned to the frequencycorresponding to the normal motor speed and furthe;` having its inputdiagonal applied to said wave generating means, a speed governingcircuit comprising circuit means operating in response to the unbalancewave eiiective at the bridge output diagonal'and the generated wave tocause a correction in the motor speed, and means comprising saidresonant arm to supply transient electrical energy to said speedgoverning circuit in response to a sudden shift in the phase of thegenerated Wave, said speed governing circuitaoperating in' response tosaid transient energy and the generated Wave to cause a furthercorrection in the motorspeed.

2. The speed regulator according to claim 1 in which said bridge is aWheatstone bridge comprising four physical arms including said resonantarm and three other arms.

3, The speed regulator according to claim 1 in which said bridgecomprises three physical arms vincluding said resonant arm and two otherarms,

said resonant arm comprising a magnetic core, an electrical Windingapplied to said core and a capacitor connected to said winding, one ofsaid two other arms being a further electrical winding said winding ofsaid first arm.

4. A speed regulator for an electric motor inf cluding a regulatingfield winding, 'comprising means to generate an alternating wave whosefrequency is proportional to the 'speed of said motor, a bridge having aresonant armV tuned to the frequency corresponding to normal motor speedand further having two pairs of conjugate terminals of which one pair isapplied to said wave generating means, a speed governing circuitcomprising a phase detector tube having its input circuit effectivelyconnected to the other pair of said terminals and its output circuitapplied to said wave generating means, and a power tube having itscontrol grid effectively connected to the output circuit of said phasedetector tube and its output circuit embodying said motor field winding,said speed governing circuit operating in response to bridge unbalancevoltage to effect a correction in motor speed, and means comprising saidresonant arm to supply transient electrical energy to the input circuitof said phase detector tube in response to a sudden shift in the Phaseof the generated Wave, said speed governing circuit operating inresponse to said transient energy to exert a compensating effect on themotor speed.

5. The speed regulator according to claim 4 which includes aregenerative circuit extending between said phase detector and powertubes to provide said motor substantially with a fiat characteristiccomprising speed versus primary variable.

6. A speed regulator for an electric motor including a regulating fieldwinding, comprising means to generate an alternating wave whosefrequency is proportional to the speed of said motor, a bridge having aresonant arm tuned to the frequency corresponding to normal motor speedand further having two pairs of conjugate terminals of which one pair isapplied to said wave generating means, a phase detector tube having itsinput'circuit connected effectively to the other pair of said terminalsand its output circuit applied to said wave generating means, a powertube having its control grid connected effectively to the output circuitof said phase detector tube and its output circuit arranged to includesaid regulating field winding, and means comprising an independentsource to introduce in the input circuit of said phase detector tube analternating wave having a predetermined magnitude and a fixed frequencyequivalent to the frequency corresponding to the normal motor speedwhereby a synchronous relation is maintained between the frequency ofthe generated wave and the fixed frequency of the independent wave.

7. A speed regulator for an electric motor including a regulating fieldwinding, comprising means to generate an alternating wave whosefrequency is proportional to the speed of said motor, a bridge having aresonant arm tuned to the frequency corresponding to normal motor speedand further having two pairs of coniugate terminals of which one pair isapplied to said wave generating means, a speed governing circuitcomprising a phase detector tube having its input circuit effectivelyconnected to the other pair of said terminalsand its output circuitapplied to said wave generating means, and a power tube having itscontrol grid connected effectively to the output circuit of said phasedetector tube and its output circuit arranged to include said regulatingfield winding, said speed governing means operating in response tobridge unbalance voltapplied to said core and coupled inductively to ageto effect correction in motor speed, means comprising an independentsource to introduce in the input circuit of said phase detector tube analternating wave having a predetermined magnitude and a fixed frequencyequivalent to the frequency corresponding to the normal motor speedwhereby a synchronous relation is maintained between the frequency ofthe generated wave and the fixed frequency of the independent wave, andmeans comprising said resonant arm to supply a transient alternatingwave to said other pair of bridge output terminals and thereby to theinput circuit of said phase detector tube in response to a sudden shiftin the phaseI of the generated wave, said speed governing meansoperatingin response to said transient wave and said generated wave to effect afurther correction in the motor speed.

B. A speed regulator for an electric motor including a regulating fieldwinding,v comprising means to generate an alternating wave whosefrequency is proportional to the speed of said motor, a bridge networkhaving a resonant arm tuned to a certain frequency corresponding tonormal motor speed so that said resonant arm is effectively a resistanceat the certain frequency, said resonant arm tending to change the valueof its effective resistance over a certain range of variation of ambienttemperature, said bridge network further having a resistance armconnected to said resonant arm and including a thermoresponsivegelementprovided with a preselected temperature coefiicient of resistance suchthat the effective resistance of said resistance arm varies in suchsense as to compensate for the changes of the effective resistance ofsaid resonant arm whereby a predetermined ratio between the effectiveresistances of said resonant and resistance arms is maintainedsubstantially constant over the certain range of ambient temperaturevariation, said bridge network also having two pairs of conjugateterminals of which one pair is applied to said wave generating means, aphase detector tube having its input circuit connected to the other ofsaid terminal pairs and its output circuit applied to said wavegenerating means, and a power tube having its control grid connected tothe output circuit of said phase detector tube and its output circuitembodying said motor field winding.

9. A speed regulator for an electric motor including a regulating fieldwinding, comprising means to generate an alternating wave whosefrequency is proportional to the speed of said motor, a bridge networkhaving a resonant arm tuned to a certain frequency corresponding tonormal motor speed so that said resonant arm is effectively a resistanceat the certain frequency,

said resonant arm tending to change the value of its effectiveresistance over a certain range of variation of ambient temperature,said bridge network further having a resistance arm connected to saidresonant arm and including a thermoresponsive element provided with apreselected temperature coefficient of resistance such that theeffective resistance of said resistance arm varies in such sense as tocompensate for the changes of the effective resistance of said resonantarm whereby a predetermined ratio between the effective resistances ofsaid resonant and resistance arms is maintained substantially constantover the certain range ofambient temperature variation, said bridgenetwork also having two pairs of conjugate terminals of which one pairis applied to said wave generating means,

a speed governing circuit comprising a phase detector tube having itsinput circuit connected to the other of said terminal pairs and itsoutput circuit applied to said wave generating means, and a power tubehaving its control grid connected to the output circuit of said phasedetector tube and its output circuit embodying said motor field winding,said speed governing circuit operating in response to bridge unbalancevoltage to effect a correction in motor speed, and means comprisg ingsaid resonant arm to introduce a transient alternating wave in serieswith the other of said terminal pairs and thereby in the input circuitof said phase detector tube when a sudden phase shift occurs in thegenerated wave, said speed governing circuit operating in response tosaid transient wave and said generated wave to effect a furthercorrection in the motor speed.

' 10. A speed regulator for an electric motor including a regulatingfield winding, comprising l means to generate an alternating wave whosefrequency is proportional to the speed of said motor, a bridge networkhaving a resonant arm tuned to a certain frequency corresponding tonormal motor speed so that said resonant arm is effectively a lresistance at the certain frequency, said resonant arm tending to changethe magnitude of its effective resonance resistance over a certain rangeof variation of ambient temperature, said bridge network further havinga resistance arm connected to said resonant arm and including athermoresponsive element provided with a preselected temperaturecoefficient of resistance such that the effective resistance of saidresistance arm varies in such sense as yto compensate for the changes ofeffective resonance resistance of said resonant arm over the certainrange of ambient temperature variation, said bridge network also havingtwo pairs of conjugate terminals of which one pair is applied to saidwave generating means, a phase detector tube having its input circuitcon-v nected to the other pair of said terminals a'nd its output circuitapplied to said wave generating means, a power tube having its controlgrid applied to the output circuit of said phase detector varies fromthe certain frequency, to cause a correction in the motor speed.

12. A speed regulator for dynamo-electric apparatus, comprising means togenerate an alternat- Y magnetic'core, an electrical winding applied tosaid core and a capacitor such that said winding and capacitorconstitute a circuit resonant at a certain frequency corresponding tonormal apparatus speed, a second arm embodies a resistive network, athird arm embodies a further electrical winding applied to said core andcoupled inductively to said winding, and conjugate pairs of input andoutput terminals of which the input terminal pair is applied to saidgenerating means, space discharge means having its input circuitconnected to said output terminal pair and its output circuit applied tosaid generating means, further space discharge means having its controlelectrode connected to the output circuit of said space discharge means,a regulating field winding for said apparatus connected in the outputcircuit of said further space discharge means, and feedback meansextending between both said space discharge means to provide saidapparatus with a substantially fiat characteristic comprising speedversus primary variable.

13. In combination in a speed regulator system, dynamo-electricapparatus having a field winding to regulate the speed thereof, agenerator to produce a pilot wave whose frequency is proportional to thespeed of said apparatus, a frequency discriminating bridge having in onearm a magnetic core, an electrical winding on said core, and acapacitor, said one arm constituting a resonant circuit tuned to acertain frequency corresponding tube and its output circuit embodyingsaid regulating eld winding, and means including an independent sourceto introduce in the input circuit of said phase detector tube in serieswith said other terminal pair an alternating wave whose frequency isfixed and equivalent to the frequency corresponding to the normal motorspeed whereby a synchronous relation is maintained between the frequencyof the generated wave and the fixed frequency of the independent wave.

11. A speed regulator for dynamo-electricl apparatus, comprising meansto generate a pilot alternating wave whose frequency is proportional tothe speed of said apparatus, a network comprising three physical arms ofwhich a first arm embodies a magnetic core, an electrical windingapplied to said core and a capacitor such that said winding andcapacitor constitute a circuit resonant at a certain frequencycorresponding to a normal apparatus speed, a second arm embodies aresistive network, a third arm embodies a further electrical windingapplied to said core and coupled inductively to said winding, andconjugate pairs of to thenormal speed of said apparatus, said bridgefurther having conjugate pairs of input and output terminals of whichthe input terminal pair is applied to 'said generator, said bridgeproviding substantiallyl no alternating wave at its outputI terminalpair when the frequency of the pilot wave is substantially equivalent tothe certainfrequency and further providing an alternating wave ofvarying phase at its output terminal pair when the frequency of thepilot wave varies with reference to the certain frequency. a phasedetector tube having its input circuit applied to saidbridge outputterminal pair and its output circuit applied tov said generator so thatthe phase relation between` the waves applied simultaneously to itsinput and output circuits determines the amount of space current fiowingin its output circuit, a power tube having its control electrode appliedto the output circuit of said phase detector tube and its output circuitarranged to include said regulating field winding so that the changes intheamount of current flowing in said latter winding are responsive tothe changes in the amount of current flowing in the output circuit ofsaid phase detector tube, and feedback means extending between both saidphase detector and power tubes and responsive to changes in the amountof current flowing `in said regulating field winding to provide saidwhich said one arm introduces a transient alternating wave across thebridge output terminal pair and thereby into the input circuit of saidphase detector tube in response to the occurrence in said system of atransient condition which so affects said generator as to changesuddenly the phaseof the pilot wave produced thereby, the phase relationbetween the transient and pilot waves present simultaneously in. theinput and output circuits of said phase detector tube serving to controlfurther the amount of space current flowing in the output circuit ofsaid phase detector tube.

15. 'I'he combination according to claim 13 in which the output circuitof said phase detector tube includes a resistor and a further capacitorin parallel for supplying filtered biasing voltage to the control gridof Said power tube, the time constant/s of L and 'E7- being of the sameorder of magnitude so that substantially no time delay intervenesbetween phase changes in the pilot Wave and changes in the amount ofcurrent flowing in said regulating field winding, where 1' is theresistance value of said winding of said one arm, L is the inductancevalue of said winding of said one arm, C is the capacitance value ofsaid capacitor, and R is resistance value of said resistor.

16. The combination according to claim 13 which includes means disposedin the output circuit of said phase detector tube to control the peakmagnitude of the space current iiowing therein and thereby the magnitudeof the current fiowing in an effective feedback circuit comprising theoutput circuit of said phase detector tube, said wave generating means,said bridge, and the input circuit of said phase detector tube such thatsubstantially no current flows in said effective feedback circuit;

17. The combination according to claim 13 in which a further resonantcircuit, comprising a further capacitor and the internal inductance ofsaid generator tuned to a preselected frequency corresponding to anapparatus speed which is less than the normal apparatus speed, isinterposed between said generator and the bridge input terminal pair tocontrol the magnitude of the pilot wave supplied simultaneously to thelatter terminal pair and the output circuit of said phase detector tube.

18. The combination according to claim 13 in which said bridge possessesan inherent phase characteristic and in which a network is eectivelyapplied to both the bridge input and output terminal pairs to attenuateharmonics of the pilot wave and further to compensate for said inherentphase characteristic of said bridge so that when the frequency of thepilot wave is above the certain frequency the waves suppliedsimultaneously to the input and output circuits of said phase detectortube are approximately degrees out of phase and so that when thefrequency or the pilot wave is below the certain frequency the wavessupplied simultaneously to the input and output circuits of said phasedetector tubes are approximately in phase.

19. The combination according to claim l3 in which the Q of theinductance comprising said core and winding thereon of said one arm isso proportioned that said bridge is effective over a frequency rangeequivalent substantially to x20 per cent of the certain frequency towhich said one arm is tuned.

20. In combination in a speed regulator system, dynamo-electricapparatus having a field winding to regulate the speed thereof, agenerator to produce a pilot wave whose frequency is proportional to thespeed of said apparatus, a frequency discriminating bridge having in afirst arm a magnetic core, an electrical winding on said core, and acapacitor, said first arm constituting a resonant circuit tuned to acertain frequency corresponding to the normal speed of said apparatus,said resonant circuit tending to change is effective direct currentresistance in response to variation in ambient temperature whereby thebridge balance at the certain frequency is correspondingly changed, saidbridge having in a second arm a resistive network including athermoresponsive element provided with a preselected temperaturecoefficient of resistance so that the effective resistance of saidsecond arm varies in such sense as to compensate for the change in theeffective resistance of said first arm whereby said firm arm remainssubstantially tuned to the certain frequency over a certain range ofvariation of ambient temperature, said bridge having in a third arm afurther electrical winding applied to said core and coupled inductivelywith said Winding and further having conjugate pairs of input and outputterminals of which the input terminal pair is applied to said generator,said bridge providing substantially no alternating wave at its outputterminal pair when the frequency of the pilot wave is substantiallyequivalent to the certain frequency and further providing an alternatingcurrent of varying phase at its output terminal pair when the frequencyof the pilot wave varies with reference to the certain frequency, aphase detector tube having its input circuit applied to the bridgeoutput terminal pair and its output circuit applied to said generator sothat the phase relation between the waves applied simultaneously to itsinput and output circuits determines the amount of space current flowingin its output circuit, a power tube having its control electrode appliedeffectively to the output circuit of said phase detector tube and itsoutput circuit arranged to include said regulating field winding so thatthe changes in the amount of current flowing in said latter Winding areresponsive to the changes in the amount of current flowing in the outputcircuit of said phase detector tube, and feedback means extendingbetween both said phase detector and power tubes to provide saidapparatus with a substantially fiat characteristic comprising speedversus primary variable.

21. The combination according to claim 20 in which said resonant circuitsupplies a transient wave to the input circuit of said phase detectortube, in response to a sudden change in the phase of the pilot wave, sothat the phase relation between said transient and pilot waves presentsimultaneously in the respective input and output circuits of said phasedetector tube determines further the amount of space current flowing inthe output circuit of said ,phase detector tube.

22. A speed regulator for an electric motor, comprising means t0generate a pilot alternating wave whose frequency is proportional to thespeed of said motor, a bridge having a resonant arm tuned to thefrequency corresponding to the normal motor speed and further having itsinput diagonal applied to said wave generating means, a speed governingcircuit comprising circuit means operating in response to both anunbalance wave effective at the bridge output diagonal and the pilotwave to cause a correction in the motor speed, means comprising saidresonant arm to supply transient electrical energy to said speedgoverning means in response to a sudden shift in asomar? the phase ofthe generated wave whereby saidv speed governing means responds to boththe transient and pilot waves to effect a further ycorrection in themotor speed, and an independent source to introduce Vin series with thebridge output diagonal an alternating wave having a Predeterminedmagnitude and a fixed frequency equivalent to the frequency to whichsaid resonant arm is tuned whereby a synchronous relation is maintainedbetween the frequencies of the pilot and independent waves.

23. A speed regulator for dynamo-electric apparatus embodying aregulating field winding, comprising means to produce a pilot wave whosefrequency is proportional to the speed of said apparatus, frequencydiscriminating means connected between said pilot means and saidregulating ileld winding and including a resonant circuit tuned to acertain frequency corresponding to the normal speed of said apparatus tovary the amount of current flowing in said regulating eld winding inresponse to changes in the frequency of the pilot wave due to variationsin the apparatus speed, and means comprising said resonant circuit tovary further the amount of current flowing in said regulating eldwinding when the phase of the pilot wave is suddenly changed.

HUGH M. SI'OILER..

